Subject: Crazing of acrylic sheeting
Thomas Dixon in Cons DistList 10:53 asked about acrylic sheet crazing. This is very common. When acrylic sheet is made it is normally cast and the thickened monomer polymerises between two sheets of glass. The shrinkage on polymerisation is significant (>10%) and as a result internal stresses are built in to the sheet. When the sheet is cut to manufacture a box or frame, etc., then the internal stress is relieved at the point of the cut but still remains in the rest of the sheet. Any slight imperfection on the surface will cause a localised concentration of those stresses and the sheet could then start to crack at that point. In practical terms this is not normally a problem unless the sheet is exposed to an aggressive environment. UV from outdoor exposure over a long time (normally years rather than days) will cause surface cracks to appear as the stress is relieved. These appear as crazing on the surface. The most common occurrence is when the sheet is exposed to non polar solvents as vapours. I have seen the acrylic front of a notice board craze in less than a week because the inside of the board had been faced with a textile which was glued into place using a solvent based adhesive. The same problem occurs when acrylic is used for outdoor sculptures. Incidentally the polystyrene jars used to package whole cloves in spice stores normally exhibit the same crazing after a few months due to the clove oil. This crazing results from the internal stress developed on manufacture and can by minimised or even prevented by annealing the final shape before it gets exposed to the environment. Annealing involves heating in an oven and then slowly cooling over several days. Needless to say, very accurate temperature control is essential. Most acrylic sheet manufacturers will have literature on annealing. Here in the UK ICI have an excellent one for their Perspex sheet. I think that extruded acrylic sheet is less prone to crazing than the cast version, it is less optically pure, but also less expensive, please note 'less' prone not 'not' prone. Colin Williamson *** Conservation DistList Instance 10:55 Distributed: Monday, December 9, 1996 Message Id: cdl-10-55-001 ***Received on Saturday, 7 December, 1996